Batteries (cells) that generate electric power through physical or chemical reaction to supply the generated power to the outside are used when AC power to be supplied to the building is not obtained, or DC power is required according to the living environments surrounded by various electric and electronic devices.
Among such batteries, primary batteries and secondary batteries, which are chemical cells using chemical reaction, are generally used. The primary batteries are consumable cells which are collectively referred to as dry cells. Also, secondary batteries are rechargeable batteries that are manufactured by using a material in a redox process between current and a substance is repeatable several times. When the reduction reaction is performed on the material by the current, power is charged, and when the oxidation reaction is performed on the material, power is discharged. Such the charging-discharging is repeatedly performed to generate electricity.
A lithium ion battery of the secondary batteries is manufactured through the following processes. An active material is applied to each of a positive electrode conductive foil and a negative electrode conductive foil at a predetermined thickness, and a separator is disposed between the positive electrode conductive foil and the negative electrode conductive foil, and then, an electrode assembly, in which the positive electrode conductive foil, the separator, and the negative electrode conductive foil are stacked, is accommodated into a prismatic can, a pouch, and the like to seal the resultant product, thereby manufacturing the lithium ion battery.
A secondary battery including an electrode for improving safety when the battery is overcharged according to the related art is disclosed in Korean Patent Publication No. 10-2007-0112717.
However, in the secondary battery according to the related art, various phenomena and changes occur in an electrode assembly during operation. Although the results obtained through real-time observation of the various phenomena and changes are capable of being utilized for diagnosis and improvement of basic research performance, it is difficult to observe the electrode assembly having a structure, in which a negative electrode, a separator, and a positive electrode are stacked, because the electrode assembly is sealed within an opaque battery case.